When a human body touches an integrated circuit, the accumulated electrostatic charges pass from the human body to the integrated circuit through the pins, and then the integrated circuit is discharged through the ground contact. The discharging process may produce a current of several amps within a few nanoseconds, leading to device degradation or damage. Therefore it is necessary to design an ESD protection system for the devices in an integrated circuit to sustain ESD events.
In monolithic compound semiconductor structures, ESD protection circuits are usually composed of p-n junction diodes or Schottky diodes due to the restriction caused by the vertically stacked layer structure. To achieve a high ESD protection level, several diodes connected in series are necessary and a larger die area is required. Besides, the turn-on voltage of a diode is small, which in turn limits the ESD protection level.
A silicon-controlled rectifier (SCR), also known as a thyristor, is a solid-state device with a four-layer pnpn structure commonly used in silicon integrated circuits. A SCR has a high turn-on voltage and a low holding voltage. When used in an ESD protection circuit, the SCR can clamp the voltage of the circuit in a lower level, such that the circuit can be effectively protected.
To achieve better device integration on a chip, a so-called BiHEMT structure that combines an HBT and an FET (or a high-electron-mobility transistor; HEMT) in a vertically stacked epitaxial structure has recently been developed for monolithic integrations of compound semiconductor devices. The HBT has either an npn or a pnp junction, while the FET/HEMT can be either n-type or p-type. If a four-layer pnpn junction can be formed in a BiHEMT structure, an epilayer structure that combines an SCR and a BiHEMT becomes possible. This can improve the ESD protection level while keeping the device area minimal.